Nucleic acids of various RNA tumor viruses have been fractionated into sequences which are shared or unique among these viruses. This allows us to examine small genetic differences between these viruses and in addition allows us to probe normal and transformed cells for the expression of these sequences. Ultimately, this work may lead to the isolation of the gene(s) responsible for the neoplastic changes induced by viruses and to identify this change in biochemical terms. The detection and characterization of virus-specific proteins in cells has been accomplished and, together with nucleic acid hybridization, allows us to examine fine structural differences between various type C viruses and the attempt to correlate these differences with oncogenic potential of viruses is ongoing.